Polycrystalline diamond (PCD) materials may be made by subjecting a mass of diamond particles of chosen average grain size and size distribution to high pressures and high temperatures while in contact with a pre-existing hard metal substrate. Typical pressures used in this process are in the range of around 4 to 7 GPa but higher pressures up to 10 GPa or more are also practically accessible. Temperatures employed are above the melting point at such pressures of the transition metal binder of the hard metal substrate. For the common situation where tungsten carbide/cobalt substrates are used, temperatures above 1395° C. suffice to melt the metal in the binder, for example cobalt, which infiltrates the mass of diamond particles enabling sintering of the diamond particles to take place. The resultant PCD material may be considered as a continuous network of bonded grains of diamond with an interpenetrating network of binder, for example a cobalt based metal alloy. The so-formed PCD material which forms a PCD table bonded to the substrate, is then quenched by dropping the pressure and temperature to room conditions. During the temperature quench, the metal in the binder solidifies and bonds the PCD table to the substrate. At these conditions, the PCD table and substrate may be considered as being in thermoelastic equilibrium with one another.
Typically, but not exclusively, cutting elements or cutters for boring, drilling or mining applications consist of a layer of polycrystalline diamond material (PCD) in the form of a diamond table bonded to a larger substrate or body often made from tungsten carbide/cobalt cemented hard metal. Such cutters with their attendant carbide substrates are traditionally and commonly made as right cylinders with the polycrystalline diamond layer or table typically ranging in thickness from about 0.5 mm to 5.0 mm but more often in the range 1.5 mm to 2.5 mm. The hard metal substrates are typically from 8 mm to 16 mm long. The commonly used diameters of the right cylindrical cutters are in the range 8 mm to 20 mm.
Other PCD constructions such as general domed and pick shaped elements are also used in various applications, for example drilling, mining and road surfacing applications. Often, the PCD material forms an outer layer on such elements with a metal carbide being used as a substrate bonded thereto. Again, the substrate is usually the largest part of such structures.
Commonly, the types of drill bit where such cutters are employed are termed drag bits. In this type of drill bit, several PCD cutters are arranged in the drill bit body so that a portion of the top peripheral edge of each PCD table bears on the rock formations. Due to the rotation of the bit, the top peripheral edge of each PCD table of each cutter experiences loading and subsequent abrasive wear processes resulting in a progressive removal of a limited amount of the PCD material. The worn area on the PCD table is referred to as the wear scar.
The performance of PCD cutters during drilling operations is determined, to a large extent, by the initiation and propagation of cracks in the PCD table. Cracks which propagate towards and intersect the free surface of a cutter may result in spalling of the cutter where a large volume of PCD breaks off from the PCD table. The result of this phenomenon may reduce the useful life of the drill bit and may lead to catastrophic failure of the cutter.
It is desirable that any cracks that form should be arrested, inhibited or deflected from propagating through the body of the PCD table to a free surface, thereby prolonging the useful life of the cutter.
International patent application WO 2004/111284 discloses a composite material comprising a plurality of cores, each core comprising a single granule of PCD, the cores being dispersed in a matrix which coats the individual granules, and a suitable binder. The matrix is formed of a PCD material of a grade different to that of the cores.
Other known solutions concern, directly or indirectly, limited ways of dealing with crack behaviour for example by means of specific layer designs.
There is a need for general solutions for a polycrystalline superhard material having favourable residual stress distributions which can ameliorate undesirable crack propagation and so lead to the reduction of spalling.